Increased cardiac contractility during acute stress is largely mediated by ?-adrenergic receptors (AR). In contrast, angiotensin (AT1) receptors appear to play the major role in cardiac adaptation to chronic hemodynamic stress. Both ?-AR and AT1 receptors are members of the superfamily of G protein-coupled receptors that are desensitized via activation-dependent phosphorylation by G protein-coupled receptor kinases (GRKs). Emerging data in cell-based systems suggest that, concurrent with desensitization of G protein signaling, GRK phosphorylation of receptors can initiate non-G protein signaling through MAP/SAP kinases. Importantly, GRK2 is postulated to be a pathological mediator of heart failure, and our studies show that a common genetic polymorphism of GRKS can determine therapeutic efficacy of p-AR blockade in African Americans with heart failure. Of the seven mammalian GRKs, GRK2 and GRKS are most highly expressed in the heart, and can modulate both ?-AR and AT1. Because deletion of the GRK2 gene results in embryonic lethality, the relative physiological importance of these two GRKs as regulators of acute and chronic cardiac adaptive responses is unclear. Thus, to better understand the roles of GRK2 and GRKS on ?-AR and AT1 signaling pathways during acute or chronic cardiac stress, we have generated mice in which GRK2 can be conditionally ablated using Cre-lox technology. While germ-line deletion with Ella-Cre was lethal, cardiac-myocyte specific deletion with Nkx2.5-Cre resulted in viable mice suitable for study. Additionally we have obtained GRKS loxP mice, and are generating the dual cardiac GRK2/GRK5 knockouts. Using these and other model systems we propose the following Specific Aims: SA#1-Delineate the individual modulatory effects of GRK2 and GRKS on the cardiac inotropic response to catecholamines and angiotensin II. SA#2-Elucidate the effects of GRK2 and GRKS on cardiac MAPkinase and SAPkinase signaling through the ?-AR and AT1 receptor pathways. SA#3-Define the signaling functions of GRK2 and GRKS during embryonic development. Our studies will use state-of-the art analytical techniques of genetic manipulation and micro-physiological analysis in cell-based systems and the in vivo mouse heart to provide insight into the roles of GRK-mediated signal modulation in cardiac stress-responses.